1. Field of the Invention
This invention relates to an improved process for producing hydroxylamine in which hydrogen sulfide is reacted with nitrogen oxide in aqueous acidic media, containing a sulfur-resistant catalyst and substantially in the absence of elemental oxygen.
2. Brief Description of the Prior Art
Hydroxylamine, NH.sub.2 OH, is a valuable intermediate in the overall synthesis of nylon-6. In one commercial process, the reagent is reacted with cyclohexanone to produce cyclohexanone oxime, which then undergoes a Beckmann-type rearrangement to form caprolactam, which is then polymerized to produce the industrially valuable polyamide, nylon-6.
Several commercial processes for manufacturing hydroxylamine are currently in use. One process that is employed, the Raschig process, is illustrated by the following equation: EQU NH.sub.4 NO.sub.3 + 2SO.sub.2 + 3NH.sub.3 + 3H.sub.2 O .fwdarw. NH.sub.2 OH + 2(NH.sub.4).sub.2 SO.sub.4
which utilizes the reduction of ammonium nitrate with a bisulfite solution of sulfur dioxide.
Another process currently in use involves the direct reduction of nitric oxide with hydrogen gas. Nitric oxide is first prepared by the catalytic reduction of ammonia with a platinum-rhodium catalyst. The formed nitric oxide is subsequently reduced with hydrogen gas in acidic aqueous media using a slurried platinum or other noble metal catalyst. The process is further described in the article, "Manufacture of Hydroxylamine by Catalytic Reduction of Nitric Oxide", appearing in Nitrogen, No. 50, pp. 27 (1967), including patent references. Use of a sulfided platinum catalyst in the above process is also described in U.S. Pat. Nos. 3,959,469 (1976) and 3,295,925 (1967).
However, the above processes require the use of industrial grade chemicals and the in situ preparation of nitrogen oxide and fail to utilize waste industrial chemicals which are produced in the chemical industry.
Two industrial waste products, hydrogen sulfide, H.sub.2 S, and nitric oxide, are currently produced in large quantities from commercial processes, and are normally discarded. Waste hydrogen sulfide is normally present in natural gas wells, which must be removed from natural gas before use, and is also obtained from petroleum refining and coal and oil cleaning operations. Under current pollution standards, more of this waste gas is expected to be generated. Waste nitrogen oxides, represented by the formula, NO.sub.x and including NO, NO.sub.2, N.sub.2 O.sub.3, and mixtures thereof, are produced for example, from nitric acid manufacture which occurs on a large industrial scale. Methods of utilizing these waste materials for manufacturing other important industrial chemicals are constantly being searched for.
The reaction between hydrogen sulfide and nitric oxide is known, but has not been studied with respect to utilizing these materials in producing other valuable industrial chemicals.
The reference, J. Phys. Chem., Vol. 35, pp. 1721-24 (1931), describes the reaction of NO with H.sub.2 S solution to yield ammonium thiosulfate, ammonium nitrite, sulfur, nitrous oxide and nitrogen. The reaction was shown to be more rapid in the presence of sulfuric acid. Pure nitrogen was produced in the process with an excess of hydrogen sulfide while nitrous oxide was formed with an excess of the nitrogen oxidizing agent. However, no hydroxylamine was detected in the reaction mixture (see p. 1721).
The reference, J. Chem. Soc. 2631-43 (1928), describes the reaction of hydrogen sulfide with nitrous acid, in the absence of a catalyst, wherein the nitrous acid is prepared from a mixture NO and O.sub.2 and is passed into a vessel containing hydrogen sulfide. When nitrous acid was in excess, nitrogen oxide and smaller quantities of nitrous oxide were produced, while variable yields of ammonia and hydroxylamine were produced when excess hydrogen sulfide was present.
We have surprisingly found that superior yields of hydroxylamine are produced when hydrogen sulfide is reacted with nitric oxide, or mixtures of nitrogen oxides, in aqueous acidic media, in the presence of a solid sulfur-resistant catalyst and substantially in the absence of elemental oxygen. The reaction is conducted in an aqueous acidic media having a pH below 7, and preferably from about 1 to 4, using a H.sub.2 S/NO.sub.x molar ratio of about 3:1, and an activated carbon catalyst.
The process thus provides a new economical route to the synthesis of hydroxylamine while at the same time providing a new use for the industrial waste materials of hydrogen sulfide and nitrogen oxide.